1. Field of the Invention
The present invention relates to a method for manufacturing an anisotropic magnet material from material having at least the three components iron (fe), boron (b) and a rare-earth metal (SE). This method performs a rapid solidification (quenching) of an alloy melt of the desired composition and subsequently treats the alloy to generate magnetic anisotropy.
2. Description of Related Art
The Nd-Fe-B magnet materials show remanence values and energy densities at room temperature which are distinctly greater than those of the known alloys on an Sm-Co basis. It should therefore be expected that these materials can replace conventional Sm-Co materials in many applications. The excellent magnetic properties of this three component system are based on the tetragonal intermetallic phase Nd.sub.2 Fe.sub.14 B. This phase, sometimes also called theta phase, has a uniaxial crystal anisotropy that gives an anisotropy field H.sub.A of approximately 75 kOe at 300.degree. K.
Anisotropic Nd-Fe-B magnet materials are frequently prepared by powder metallurgy (see European Patent Application No. 0 126,179). According to this method, an alloy of the desired composition is first ground until the powder grains have the size of single domain (span) particles between 2 and 4 .mu.m. The powder grains are then aligned in a magnetic field, precompacted by isostatic pressing and then sintered to form a high-density body. The magnetic properties are then optimized by a final heat treatment.
An alternative method, such as disclosed in European Patent Application No. 0,144,112, makes isoptropic tapes of the desired composition by first rapidly solidifying an alloy melt of the desired composition. These tapes have a fine-crystalline structure. The tapes are then compacted by compression at temperatures of about 700.degree. C. to form a dense isotropic body. Subsequent hot deformation at about 700.degree. C. by approximately 50% leads to an anisotropic texture with the c-direction being the magnetically easy axis parallel to the direction of the pressure (see also "Appl. Phys. Lett." 46 (8), Apr. 15, 1985, pages 790 and 791).
Technical magnets have a general composition of the type Nd.sub.15 Fe.sub.77 B.sub.8 and are in a three-phase equilibrium between the magnetically hard Nd.sub.2 Fe.sub.14 B-phase, a phase Nd.sub.1.14 Fe.sub.4 B.sub.4 -phase rich in B, and a solid solution (mixed crystal) rich in Nd. The foreign phases are necessary here in part to optimize the structure-dependent coercitive properties.
Despite their superior magnetic properties, the use of Nd-Fe-B materials is strictly limited by its low Curie temperature T.sub.c of about 315.degree. C. The remanence and coercivity field strengths drastically decrease with increasing temperature and fall below the values of optimized Sm-Co magnet material. Attempts have been made to increase the Curie temperature by partially substituting Fe with Co so as to increase the span between the Curie and the use temperature (see "Appl. Phys. Lett." 46 (3), Feb. 1, 1985, pages 308 to 310). However, the results on sintered magnets show that Co additions cause the coercitivity field strength to decrease at the same time. Ultimately, the Co substitution produces no improvement. (see "IEEE Transactions Magn.", MAG-21, 1985, pages 1952 to 1954).